CN111490154A - Device comprising a tunable tetragonal ferrimagnetic heusler compound - Google Patents

Abstract

A device comprising a tunable tetragonal ferrimagnetic heusler compound is disclosed. The device comprises Mn_3‑xCo_xHeusler compounds of the Ge form, where 0<x is less than or equal to 1, wherein Co accounts for at least 0.4 atomic percent of the heusler compound. The device further comprises a layer oriented in the (001) direction and in YMn_1+dA form of the substrate, wherein Y comprises an element selected from the group consisting of Ir and Pt, and 0 ≦ d ≦ 4. Tetragonal heusler compoundsAnd the substrate are in proximity to each other such that a spin-polarized current passes from one of the tetragonal heusler compound and the substrate through the other of the tetragonal heusler compound and the substrate. In one aspect, the device further comprises a multilayer structure that is non-magnetic at room temperature. The structure includes alternating layers of Co and E. E includes at least one other element including Al. The composition of the structure is Co_1‑yE_yAnd y is in the range of 0.45 to 0.55.

Description

Device comprising a tunable tetragonal ferrimagnetic heusler compound

This application claims the benefit of a U.S. patent application serial No. 16/260,024, filed by the united states patent and trademark office on 28/1/2019, the entire contents of which are incorporated herein by reference.

Technical Field

The invention relates generally to a tunable tetragonal ferrimagnetic Heusler compound (tunablet coarse aluminum ferromagnetic Heusler compound) with PMA and high TMR.

Background

In today's Magnetic Random Access Memory (MRAM), the basic storage element is a Magnetic Tunnel Junction (MTJ) consisting of two magnetic layers separated by an ultra-thin insulating layer called a "tunnel barrier". The resistance of the MTJ depends on the relative orientation of the magnetizations of the two magnetic layers. The magnetization, referred to as the storage layer or memory layer, switches between being parallel or anti-parallel to the magnetization of the reference magnetic layer. Currently, the change in magnetic state of an MTJ is achieved by passing a current through the device. In today's MRAMs, the magnetic electrodes of the MTJ are formed of ferromagnetic alloys of Co, Fe, and B, with their magnetic moments oriented perpendicular to the layers. This perpendicular orientation of the magnetization of the CoFeB alloy results from interface effects, which are generally weak to limit the MTJ size to ≧ 20 nm. It is necessary to find alternative magnetic materials for use in MTJs that have Perpendicular Magnetic Anisotropy (PMA) due to their bulk properties.

Disclosure of Invention

According to one aspect of the invention, there is provided a ferrous iron comprising a tunable tetragonal ferriteA device of magnetic heusler compounds, the device comprising: is Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x ≦ 1, wherein Co is at least 0.4 atomic percent of the Hasteller compound, and the Hasteller compound has a tetragonal structure; and a substrate oriented in the (001) direction and in YMn_1+dA form wherein Y comprises an element selected from the group consisting of Ir and Pt, and 0 ≦ d ≦ 4, wherein the Hausler compound and the substrate are proximate to each other, thereby causing a spin-polarized current to pass from one of the Hausler compound and the substrate through the other of the Hausler compound and the substrate.

According to another aspect of the invention there is provided a device comprising a tunable tetragonal ferrimagnetic heusler compound, the device comprising: is Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x is less than or equal to 1, wherein Co accounts for at least 0.4 atomic percent of the Hasteller compound, and the Hasteller compound has a tetragonal structure; a multilayer structure, the multilayer structure being non-magnetic at room temperature, the multilayer structure comprising alternating layers of Co and E, wherein E comprises at least one other element comprising Al, wherein the composition of the multilayer structure consists of Co_1-yE_yAnd y is in the range of 0.45 to 0.55; and a substrate positioned under the multilayer structure, wherein the heusler compound and the multilayer structure are in proximity to each other such that a spin-polarized current passes from one of the heusler compound and the multilayer structure through the other of the heusler compound and the multilayer structure.

According to another aspect of the invention there is provided a device comprising a tunable tetragonal ferrimagnetic heusler compound, the device comprising: a substrate; a bottom layer oriented in a (001) direction, the bottom layer being non-magnetic at room temperature, the bottom layer covering the substrate; a first magnetic layer comprising Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x is less than or equal to 1, and the first magnetic layer is in contact with the bottom layer, wherein the magnetic moment of the first magnetic layer is switchable; a tunnel barrier covering the first magnetic layer; and a second magnetic layer in contact with the tunnel barrier.

Drawings

Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.

Fig. 1A-1C depict schematic diagrams of crystal structures.

FIG. 2 depicts ordered Mn_3-xCo_xThe electron band structure of the Ge compound.

FIGS. 3A-3F depict Mn for an exemplary embodiment₃Ge/MgO/Fe MTJ and Mn_2.75Co_0.25Transmission and tunneling magnetoresistance of Ge/MgO/FeMTJ.

FIG. 4 depicts Mn of examples_3-xCo_xExperimental and theoretical magnetizations for Ge compounds.

Fig. 5A and 5B illustrate X-ray diffraction measurements for the device of the exemplary embodiment.

Fig. 6A and 6B illustrate out-of-plane (out-of-plane) and in-plane (in-of-plane) magnetic properties of devices of example embodiments.

Fig. 7 shows X-ray diffraction measurements for the device of the exemplary embodiment.

Fig. 8A and 8B depict an MRAM element of an exemplary embodiment.

Detailed Description

The invention may be embodied as a process; equipment; a system; and/or the composition of matter. In this specification, these embodiments, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention.

The following provides a detailed description of one or more embodiments of the invention and the accompanying drawings that illustrate the principles of the invention. The invention is described in connection with these embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

In some embodiments, a device includes a tetragonal Heusler compound (also referred to herein simply as Heusler or Heusler) and a substrate. The tetragonal heusler compound is Mn_3-xCo_xForm Ge, wherein 0<x is less than or equal to 1, wherein Co accounts for at least 0.4 atomic percent of the heusler compound. The substrate is oriented in the (001) direction and is YMn_1+dForm wherein Y comprises an element selected from the group consisting of Ir and Pt, and 0 ≦ d ≦ 4. The tetragonal heusler compound and the substrate are in proximity to each other such that a spin-polarized current passes from one of the tetragonal heusler compound and the substrate through the other of the tetragonal heusler compound and the substrate. In one aspect, Y is Ir. In one embodiment, the tunnel barrier is in contact with a tetragonal heusler. The tunnel barrier may include Mg and O. In some embodiments, the device further includes a TaN layer located between and in contact with the tetragonal heusler compound and the substrate. In some embodiments, the tetragonal heusler compound is Mn_3-xCo_xForm Ge, wherein 0<x is less than or equal to 1 and the substrate is IrMn₃Form (a). In some embodiments, the magnetization of the hassler compound is perpendicular to the plane of the substrate. The thickness of the heusler compound may be at least 10 angstroms and not more than 500 angstroms, and preferably, the heusler compound has a thickness of less than 5nm (e.g., a thickness of less than 3 nm) or has a thickness of one unit cell. In some embodiments, the TaN layer underlies and is in contact with the substrate. In some embodiments, the device may be used in a memory element.

In some embodiments, a device includes a quad heusler, a multilayer structure, and a substrate. The tetragonal heusler compound is Mn_3-xCo_xForm Ge, wherein 0<x is less than or equal to 1, wherein Co accounts for at least 0.4 atomic percent of the heusler compound. The multilayer structure is non-magnetic at room temperature. The multilayer structure comprises alternating layers of Co and E, wherein E comprises at least one other element comprising Al, and wherein the composition of the multilayer structure consists of Co_1-yE_yWherein y is in the range of 0.45 to 0.55. The tetragonal heusler compound and the multilayer structure are in proximity to each other such that a spin-polarized current passes from one of the tetragonal heusler compound and the multilayer structure through the other of the tetragonal heusler compound and the multilayer structure. In some embodiments, the device includes a tunnel barrier overlying the tetragonal heusler compound, allowing current to pass through both the tunnel barrier and the tetragonal heusler compound. The tunnel barrier may include Mg and O. In some embodiments, E is an AlGe alloy. In some embodiments, the device may be used as a memory element.

In some embodiments, a device includes a substrate, an underlayer, a first magnetic layer, a tunnel barrier, and a second magnetic layer. The underlayer is oriented in the (001) direction and is non-magnetic at room temperature. The first magnetic layer comprises a heusler compound Mn_3-xCo_xGe, wherein 0<x is less than or equal to 1. The first magnetic layer is in contact with the underlayer, and a magnetic moment of the first magnetic layer is switchable. The tunnel barrier covers the first magnetic layer. The second magnetic layer is in contact with the tunnel barrier. In some embodiments, the device includes a capping layer in contact with the second magnetic layer. In some embodiments, the magnetic moment of the first magnetic layer is perpendicular to an interface between the tunnel barrier and the first magnetic layer.

For several Mn_3-xCo_xThe structure and magnetic properties of Ge heusler compounds were studied theoretically and experimentally. As the Co content increases, the structure changes from tetragonal to cubic and from ferrimagnetic to ferromagnetic at higher Co content. Thus, an intermediate composition with low magnetization and PMA may be advantageous for STT-MRAM applications. Our theoretical study showed that at Co concentration x_c<x≤1、x_c(ii) a Brillouin zone filtering effect is inhibited in compounds in the range of about 0.2, the Brillouin zone filtering effect being inMn of perpendicular-magnetized MTJ₃A compensated Tunneling Magnetoresistance (TMR) at the Ge/MgO interface works. Therefore, the compensation between the Spin Polarization (SP) effect and the BZF effect can be eliminated so that the MgO thickness (N) for the same range_MgO8-12), Mn with x-0_3-xCo_xTMR (TMR) for Ge/MgO/Fe MTJs<100%) compared with 0.25 of Mn, x ═ 0.25 of Mn_3-xCo_xThe TMR (TMR about 500%) of the Ge/MgO/Fe MTJ is significantly increased.

Computational research

A. Ordered Mn with x equal to 0, 0.25, 0.5, 1, 1.5 and 2_3-xCo_xCrystal and magnetic structure of Ge compound

FIGS. 1A-1C depict tetragonal-Mn₃Ge (FIG. 1A), tetragonal-Mn_2.5Co_0.5Ge (FIG. 1B) and cubic-MnCo₂Crystal structure of Ge (fig. 1C). a is_tet、c_tetRepresents the tetragonal lattice constant, a_cubRepresenting the cubic lattice constant. The labels "T" and "O" represent sites tetrahedrally and octahedrally coordinated by Ge atoms, respectively. The tetragonal unit cell can rotate in a 45-degree plane

And elongated along the c-axis to obtain a cubic unit cell. Note that the orientation of the Mn and Co magnetic moments is chosen so that the magnetic moment of the unit cell is positive along the-C axis in FIGS. 1A and 1B and along the + C axis in FIG. 1C. The sizes of the atoms are not drawn to scale.

To find various Mn_3-xCo_xWhether the ground state structure of Ge compounds is cubic or tetragonal, and whether it is positive or negative and the corresponding lattice constant and magnetic configuration, we perform the calculations using a generalized gradient approximation in the theory of density functional implemented in the VASP program employing projected decorated wave (project augmented wave) potentials. The lowest energy configurations for x ═ 0, 0.25, 0.5, 1, 1.5, 2 were found from total energy calculations performed for various atomic orderings of Mn and Co atoms within the unit cell. Cell size varies according to x: for x ═ 0, 1, and 2, the unit cell has 4 atoms; for x ═ 0.5 and 1.5, the unit cell has 8 atoms; for x ═ 0.25, unit cellThere are 16 atoms [ in subsection A we consider only ordered compounds, while in subsection D disordered compounds are considered by using a virtual crystal approximation]For compounds with 4 atoms in the unit cell, a preliminary rough estimate of the lattice parameters was made using a 6 × 6 × 6 k-point grid, and fine adjustments were made to the lattice parameters using a 10 × 10 × 10 k-point grid with a truncation energy equal to 400 eV.

Table I summarizes the ground state structure and the corresponding magnetic states for both cubic and tetragonal structures. In some cases, no metastable tetragonal state was found. The minimum energy configuration was found to be Mn₃Tetragonal system of Ge, Mn₂Inverse tetragonal system of CoGe and MnCo₂A positive cubic system of Ge. The minimum energy configuration for cells with 8 and 16 atoms in the cell was found to be a stack of several 4 atom cells along the z-axis. Thus, for x equal to 0.5, the ground state is centered at tetragonal Mn₃Inverse tetragonal Mn above Ge Single cell₂CoGe unit cell. For x 0.25, the ground state is composed of three tetragonal Mn oriented in the z direction₃Inverse tetragonal Mn above Ge Single cell₂CoGe unit cell. Finally, for x ═ 1.5, the ground state is composed of orthorhombic MnCo₂Reverse cubic Mn above Ge Mono cells₂CoGe unit cell. Calculations show that for x changes up to and including 1, the ground state is still tetragonal. For x>The ground state becomes cubic 1.

Table I, structure type, lattice constant, stabilization energy, total magnetic moment per formula unit (in terms of 4 atoms) and magnetic moments m of individual Mn and Co atoms. When highlighted in bold, a single m value represents a Co atom, otherwise a Mn atom. Magnetic moment m_(O)And m_(T)Represents a single atom in the designated 4 atom unit cell in both octahedral coordination (layers with main group Ge atoms) and tetrahedral coordination.

For the cubic phase, it was found that the magnetic moments satisfy the Slater-Pauling rule for all values of x that we studied. The sleet-bowlin rule states that the magnetic moment m per 4 atomic unit cell depends on the number of valence electrons N per 4 atomic unit cell_VChange is that m is equal to N_V-24. The single calculated magnetic moments of the Mn and Co atoms at the O site and at the two T sites are shown in table I. Note that for all compositions considered here, the O sites (in both the cubic and tetragonal phases) are preferentially occupied by Mn according to the "lightest atom" rule, which indicates that in the cubic X₂The O site in YZ hassler compounds should always be occupied by an X atom or a Y atom having a low valence. Furthermore, the magnitude of the magnetic moment of the Mn atom on the O site is robust (robust) and shows only small changes with Co composition. This indication of the magnetic moment varies depending on whether the magnetic moment is dominant for the Ferrimagnetic (FiM) case. The magnetic moments of the Mn and Co atoms at the T site change significantly from cubic to tetragonal, but for each of these phases they have similar values independent of Co content within the corresponding 4-atom unit cell building block. In both cubic and tetragonal phases, the magnetic moment of a Co atom (which is always on the T site) is always oriented ferromagnetically with the Mn magnetic moment on the O site, while the magnetic moment of a Mn atom on the T site is always oriented antiparallel to the Mn magnetic moment on the O site. When Co is added, it replaces the Mn atom on the T site, so that when x ═ 2 there is no more Mn atoms on the T site and the magnetic configuration becomes Ferromagnetic (FM). In the cubic phase, Mn₃The net magnetic moment (net magnetic moment) of Ge has the same sign as the Mn magnetic moment at the O site. On the other hand, in the tetragonal phase, Mn₃The total magnetic moment of Ge is of opposite sign to the Mn magnetic moment on the O site. Thus, the addition of Co (again at the T site) initially reduces the total magnetic moment until it is zero at x ═ 0.5 (fig. 1B). Further increases in Co content then cause an increase in the total magnetic moment (this was also experimentally observed in our sputter-deposited films)A trend).

From the calculated partial density of states (pDOS) of the different electronic structures, we can extract the layer-dependent SPs defined as follows:

SP＝[pDOS(maj)-pDOS(min)]/[pDOS(maj)+pDOS(min)](1)，

wherein pDOS (maj) and pDOS (min) are at Fermi energy (E), respectively_F) The calculated majority and minority spin-polarized fraction DOS for each layer in the unit cell. Note that each layer has two atoms as shown in table II. Here, the calculated SP is shown only for the ground state configuration. Regardless of the Co content, all cubic phases are semimetallic (SP ═ 1). Note that for the compounds shown in table II, the SP of each layer in the unit cell has the same sign for all compositions and phases, and it always corresponds to the sign of the magnetic moment of the Mn atom on the O site (i.e., Mn — Ge layer). Thus, the polarization changes sign at x-1.

Table II bulk layer-by-layer polarization at different Co concentrations. Note that for x ═ 0, 1, 2, there are only 2 repeat layers. A low energy configuration with lattice parameters from table I was used for each x. The z-position of each layer corresponds to its position along the C-axis (see fig. 1A-1C).

B. Brillouin zone spin filtering effect

If the Magnetic Electrode (ME) is in one spin channel sigma₁Middle edge-Z line at E_FHas an electron state and is in another spin channel σ₂Middle edge-Z line at E_FHas no electron state, so-called Brillouin zone spin filtering (BZF) effect occurs in the ME/MgO/ME MTJ system. the-Z line in the Brillouin Zone (BZ) is along the in-plane wave vector k_||＝k_x,k_y) K of 0_zA line of direction. As is well known in the artMinimum of MgO insulating spacers (at a given k) with evanescent states within the MgO bandgap as energy propagates in the z-direction_||) Attenuation constant gamma (k)_||) At k_||Reaches a minimum of gamma at 0₀. When k is_||When increasing, γ (k)_||) Is increased to

Wherein, α>0(α is small k_||Taylor expansion coefficient). Thus, considering ME as described above, propagation in the z-direction is in E in spin channel σ 1_FAt cryptomorphic state of_||When 0, the crystal will be in MgO with exp-gamma₀z]Decay due to k_|| Spin channel σ 2 at E when 0_FHas no electron state, and is therefore in E in spin channel σ 2_FWill follow k with hidden state_||>0 to

And (4) attenuation. As a result, TMR of ME/MgO/ME MTJs varies with MgO thickness d_MgOIncrease in (a) is exponential, e.g.

Wherein the content of the first and second substances,

is ME at E in spin channel σ 2_FHas the smallest k of the state_||Not equal to 0 vectors. TMR pair d_MgOIs less than that in a conventional Fe/MgO/Fe MTJ from a symmetric spin-filtering effect

Is stronger, wherein for MTJ architectures with square symmetry in the xy plane, the power coefficient n can only assume three values n-0, 1, 2. For simplicity we assume that both electrodes are made of the same ME material, but typically only one electrode with the above properties is sufficient to predict TMR versus d due to the BZF effect_MgOIs increased.

C. Ordered Mn at x ═ 0, 0.25, 0.5, 1, 1.5, and 2_3-xCo_xbZF effect in Ge compounds

FIG. 2 shows Mn for ordered bodies having the structure described in Table I_3-xCo_xMajority and minority electron bands of Ge compounds along-Z line in three-dimensional (3D) BZ for ordered body Mn_3-xCo_xGe compounds have a tetragonal lowest energy configuration at x 0, 0.25, 0.5, 1 and a cubic lowest energy configuration at x 1.5, 2. The band structure was calculated using the VASP program with projected infix plus wave (PAW) potentials and Perew-Burke-Ernzerh (PBE) Generalized Gradient Approximation (GGA)/DFT.

Bulk Mn₃Ge (x ═ 0) and Mn₂CoGe (x ═ 1) has a cross-over E along the-Z line_FOf (a) and (d) of fig. 2), while the minority states (g and (j) of fig. 2) do not cross E along the-Z line_FAny of the belts of (a). Therefore, these compounds satisfy the BZF condition described above, and use MgO tunnel barrier and Mn₃MTJ device formed by Ge should be larger than d as one of electrodes_MgOShows exponentially increasing TMR (see next section D). In contrast, the other two tetragonal compounds (x ═ Mn of 0.25 and 0.5) were_3-xCo_xGe) has a cross-Z line across E in both majority and minority spin channels_F(b) - (c) of fig. 2 and (h) - (j) of fig. 2), and thus these compounds do not satisfy the BZF condition (as illustrated in table II).

Prediction of Mn_1.5Co_1.5Ge (x ═ 1.5) compounds and MnCo₂Ge (x ═ 2) compounds have a cubic ground state configuration of semimetallic nature (100% spin polarization per layer, as shown in table II). These compounds have energy gaps not only at EF for wave vectors k along-Z line (as shown in fig. 2 (b) - (D)), but also for all k wave vectors in 3D-BZ.

D. Disordered Mn with x less than or equal to 0.25_3-xCo_xBZF effect and DOS in Ge Compounds

Using a full-potential linear pill-box trajectory (L MTO) method employing Barth-Hedin local density approximation (L DA)/DFT functionVirtual Crystal Approximation (VCA) in a framework simulates disordered Mn with a small value of 0 ≦ x ≦ 0.25_3-xCo_xA Ge compound. In this approximation, ordered Mn₃Nucleus Z of a Mn atom of the Ge system_MnThe charge of 25 being replaced by the charge of a "dummy" atom

Z＝(1-x/3)Z_Mn+(x/3)Z_Co(3)，

It simulates the presence of disordered Mn_3-xCo_xThe x/3 fraction of Mn sites in the Ge system is substituted by Co atoms (Z)_Co27) are randomly occupied. VCA calculation Using tetragonal Mn from previous studies₃Experimentally determined lattice constants of Ge Hashler Compounds

To be executed.

The majority and minority electronic bands along the-Z line are calculated for x 0.00, 0.05, 0.10, 0.15, 0.20 and 0.25. It was found that E was not crossed along the-Z line only for x of 0.00, 0.05, 0.10 and 0.15_FAnd for x equal to 0.20 and 0.25, one crosses E along the-Z line_FThe minority bands of (a). Thus, for ordered and disordered Mn_3-xCo_xCalculations for both Ge systems show that the BZF effect is only for small x<x_c(with estimation)

) This occurs. Further, Mn calculated in VCA for x ═ 0.00, 0.05, 0.10, 0.15, 0.20, and 0.25_3-xCo_xThe majority and minority DOS of Ge compounds indicate that a large negative SP remains for all x considered here.

E. Mn with x ═ 0 and x ═ 0.25_3-xCo_xTMR calculation for Ge/MgO/Fe System

To investigate the cause of Mn_3-xCo_xPresence and absence of BZF effect at Ge/MgO interface for x<x_cAnd x>x_cIn different environments of TMR behavior, we performed Mn of x-0 and x-0.25 using the linear pill-box orbit method (L MTO-ASA) in a tightly bound atomic sphere approximation with local density approximation of DFT for exchanging correlated energies_3-xCo_xCalculation of the transmission function of the Ge/MgO/Fe MTJ system. For the case where x is 0.25, we use VCA as described above.

Regarding Mn₃Ge/MgO/Fe MTJ with in-plane lattice constant determined for bulk tetragonal Mn₃Experimental lattice constant of Ge

Determination of Mn Using VASP molecular dynamics program₃Relaxation position of the atoms at the Ge/MgO interface (for both the Mn-Mn end and the Mn-Ge end). The O-top configuration was found to be the most stable configuration for these two ends (compared to Mg-top and hollow). For the MgO/Fe interface, atomic positions from the literature are used. For Mn_2.75Co_0.25The Ge/MgO/Fe MTJ system uses these same atomic positions. Finally, the number N of MgO layers_MgOVarying from 2 to 12.

Fig. 3A-3F depict (x ═ 0) Mn with Mn-Mn (fig. 3A, 3D) and Mn-Ge (fig. 3B, 3E) ends at the heusler/MgO interface₃Ge/MgO/Fe MTJ and (x ═ 0.25) Mn_2.75Co_0.25Ge/MgO/Fe MTJ at E_FWith the calculated transmission function (T) as N_MgOAs a function of (c). Four functions are given for each end and two x values: t [ Fe (↓) Heusler (heel)]Transmission sum T [ Fe (↓) Heusler (↓)]Transmission and T [ Fe (↓) Heusler (↓) for antiparallel configuration]Transmission sum T [ Fe (↓) Heusler (≠)]And (4) transmission. Mn₃Ge/MgO/Fe (FIG. 3C) MTJ and Mn_2.75Co_0.25TMR and N of Ge/MgO/Fe (FIG. 3F) MTJ_MgOAnd (4) oppositely. TMR having Mn-Mn (Mn-Ge) ends are shown in square symbols (triangles), while TMR considering the number of Mn-Mn ends and Mn-Ge ends being equal are shown in circular symbols. The case where x is 0.25 was calculated using VCA. FIGS. 3A and 3B show Mn₃For Mn with Mn-Mn and Mn-Ge ends at the Ge/MgO interface₃Ge/MgO/Fe MTJ at E_FWith the calculated transmission function (T) as N_MgOAs a function of (c). Four functions are given for each end: t [ Fe (≈) Mn) in parallel configuration (P)₃Ge(↑)]Transmission sum T [ Fe (↓) Mn₃Ge(↓)]Transmission and T [ Fe (≠) Mn) in antiparallel configuration (AP)₃Ge(↓)]Transmission sum T [ Fe (↓) Mn₃Ge(↑)]Transmission (majority and minority spin channels per electrode are indicated by up and down arrows, respectively). As seen in fig. 3A and 3B, the transmission through the Fe majority channel is much greater than the transmission through the Fe minority channel for both the P and AP configurations and for both the Mn-Mn and Mn-Ge ends. This is a result of the well-known symmetric spin-filtering effect produced at the Fe/MgO interface. The entire Mn is due to the major contribution of most Fe electrons to both P transmission and AP transmission₃TMR sign and size of Ge/MgO/Fe MTJ system are determined by the Mn₃The sign and magnitude of the tunneling spin polarization of the Ge/MgO interface. The latter, in turn, is formed by increasing N_MgOAnd BZF effect favorable to positive TMR and body Mn favorable to negative TMR₃The balance between the large negative SP of Ge determines. As seen in FIGS. 3A and 3B, T [ Fe (≠) Mn₃Ge(↓)]Transmission (square symbols) as N_MgOThe function decays exponentially, T [ Fe (×) Mn₃Ge(↓)]The transmission (square symbols) has a ratio of T [ Fe (×) Mn to both Mn-Mn and Mn-Ge ends₃Ge(↑)]The attenuation constant of the transmission function (circular sign) is larger (steeper slope). This is due to the BZF effect-crossing E at the-Z line along BZ_FBulk Mn of₃There is no band in the minority channel of Ge, and in bulk Mn₃Such bands are present in most channels of Ge.

TMR for both ends as N_MgOThe function of (C) is shown in fig. 3C. Here, TMR is defined as:

TMR＝(T_P–T_AP)/min(T_P,T_AP) (4)，

wherein, T_P＝T[Fe(↑)Mn₃Ge(↑)]+T[Fe(↓)Mn₃Ge(↓)]And T_AP＝Fe(↑)Mn₃Ge(↓)]+T[Fe(↓)Mn₃Ge(↑)]Respectively P configuration and AP configuration at E_FThe total transmission of (c). Qualitatively, the behavior of TMR given in fig. 3C can be explained as follows. When Mn is present₃The native spin polarizability of Ge is negative and comparable in absolute values of both the Mn-Mn end layer and the Mn-Ge end layer (see table II), when considering only the projection to the atomic rail with m-0The Mn-Ge layer exhibits a much larger absolute value of negative SP (where m is the z-axis projection of angular momentum) than the Mn-Mn layer at DOS of the trace. Due to k near the point_||The evanescent states of MgO with the smallest decay constant consist mainly of the m ≠ 0 track (and since these states tend to bind more strongly to the m ≠ 0 track of the heusler-end layer than to the m ≠ 0 track due to symmetry constraints), so for the same N_MgOIn particular, with Mn having Mn-Mn terminals₃The large difference in SP projected to DOS of m-0 orbit for Mn-Ge layer and Mn-Mn layer compared to TMR of Ge/MgO/Fe MTJ favors Mn with Mn-Ge end₃Larger negative TMR of Ge/MgO/Fe MTJ. Indeed, for all N considered here_MgOIn particular, FIG. 3C shows that the TMR calculated for the Mn-Ge end (triangle symbols) is negative, while it is positive (N) for the Mn-Mn end case (square symbols)_MgO>2). Because the BZF effect becomes stronger as the barrier thickness increases, the positive TMR increases in the case of Mn — Mn terminals, while the negative TMR becomes less negative in the case of Mn — Ge terminals, and at a sufficiently large N_MgOIt will eventually change sign.

In practical MTJ devices, Mn₃The morphology of the Ge heusler layer will inevitably fluctuate in atomic scale, causing regions with Mn-Mn and Mn-Ge ends that will interact with the MgO barrier. The simplest way to model this fluctuation is to average the transmission function for different ends (P and AP respectively) assuming the same MgO thickness throughout the device. The TMR calculated from this simple model assuming equal areas occupied by the Mn-Mn ends and the Mn-Ge ends is also shown in FIG. 3C (circle symbols). T due to Mn-Ge terminal_PAnd T_APBoth are compared to Mn-Mn end (consider all N_MgO) T of_PAnd T_APBoth are large and therefore the calculation gives a negative TMR.

FIGS. 3D and 3E show the E at both the Mn-Mn end and the Mn-Ge end, respectively_FUsing disordered Mn_2.75Co_0.25VCA calculated Mn of Ge Compounds_2.75Co_0.25The transmission function of the Ge/MgO/Fe MTJ system (for simplicity, we continue to use the "Mn-Ge" and "Mn-Mn" symbols for the end layers with and without Co atoms, respectively). Can be used forIt is seen that the major contributions to P-transmission and AP-transmission come from most Fe channels, similar to the case where x is 0. Another similarity is that the corresponding TMR shown in FIG. 3F is positive for the Mn-Mn terminus (for N)_MgO>2) And is negative for the Mn-Ge terminus. However, due to the inhibition of the BZF effect at x ═ 0.25, Mn₃Ge case and Mn_2.75Co_0.25The main difference between the Ge cases is T [ Fe (≈ Mn) ° C)_2.75Co_0.25Ge(↑)]Attenuation constant of transmission function and T [ Fe (°) Mn_2.75Co_0.25Ge(↓)]The decay constants of the transmission functions are similar (for both the Mn-Mn end and the Mn-Ge end). In the latter case, the positive TMR at the Mn-Mn terminal is at N_MgOA maximum of about 200% is reached at 10 and N_MgOStarts to fall off at 12 (fig. 3F, square symbols), while the TMR at the Mn — Mn end is much larger (at the maximum N considered) in the case of x 0 (fig. 3C, square symbols) due to the BZF effect_MgO(i.e., N)_MgOTMR about 550%) and with N ═ 12)_MgOSteadily increasing. Since there is no compensation between the BZF mechanism and the negative SP for the case of x ═ 0.25, the absolute value of the (negative) TMR at the Mn-Ge end is greater than 800% (fig. 3F, triangle symbols) for all considered MgO thicknesses, and in the case of equal area occupied by the Mn-Mn end and the Mn-Ge end, for all N considered here_MgOThe absolute value of TMR is greater than 350% (FIG. 3F, circle symbol), for N_MgOMaximum TMR is about 1000% for 4. Therefore, for the same MgO thickness range (N)_MgO8-12), and Mn of 0 for x_3-xCo_xTMR (TMR) for Ge/MgO/Fe p-MTJ<100%) the elimination of the compensation between the SP and BZF effects does result in an Mn of 0.25 for x_3-xCo_xTMR (TMR) for Ge/MgO/Fe p-MTJ>500%) was observed.

Experimental study

Thin film growth

At a reference pressure of about 4 × 10^-10The samples were prepared in an ultra-high vacuum magnetron sputtering chamber at Torr. The film stacks used in this study were as follows:

Ta。TaN/IrMn₃the bottom layer of the bilayer serves to promote the (001) crystal orientation of the heusler film, and the thin Ta cap serves as a protective layer. At Ar and N₂IrMn is induced by a TaN layer formed by reactive magnetron sputtering of a Ta target in a gas mixture₃When deposited directly on amorphous SiO₂On the surface, the (111) orientation will be favored. IrMn deposition by ion beam sputtering using Kr₃A film and a Ta cap layer. By sputtering of a single Mn₃Ge target (x ═ 0) or by Co-sputtering Mn target, Co target and Co₁₀Mn₄₅Ge₄₅Target Mn growth Using a three step Process at 3mTorr (Using Ar)_3-xCo_xGe(0<x is less than or equal to 2) film: deposition at 450 ℃ initially is

Mn of (2)_3-xCo_xA Ge layer, then deposited at 150 DEG C

Mn of (2)_3-xCo_xGe layer, and finally annealing in situ for 1-2h at 450 ℃ under vacuum. All other layers were deposited at Room Temperature (RT). All films were smooth with a root mean square surface roughness of less than

While retaining Co₁₀Mn₄₅Ge₄₅When the power of the magnetic control gun is constant, the power of the Mn and Co magnetic control guns is changed to adjust Mn_3-xCo_xRatio between Mn and Co in Ge films.

Measuring method

The composition of the films was measured by the Rutherford Backscattering Spectroscopy (RBS) measurement method, while the structure of the films was studied by the X-ray diffraction (XRD) measurement method using Bruker D8 General Area Detector Diffraction System (GADDS) irradiated with CuK α Atomic Force Microscope (AFM) measurement method was performed using Bruker Icon dimensions with Scanasyst system to determine the sample surface roughness.

Structural and magnetic Properties

FIG. 4 summarizes Mn_3-xCo_xExperimental magnetization M (open symbols with lines) and calculated magnetization M (filled symbols and open symbols without lines) for Ge compounds. Squares and triangles correspond to cubic and tetragonal structures, respectively, while circles correspond to mixed phase compounds. For each calculated Mn_3-xCo_xGe compounds, showing theoretical M values for the ground and high energy states (both present). Consistent with theoretical predictions, we experimentally observed that as the Co content increased, the crystal structure changed from tetragonal to cubic, and the overall magnetic moment was consistent with the change from FiM to the FM configuration (open symbols with lines). Although the experimental M values for films with x ═ 0.6 and 0.8 appear to be inconsistent with theory, Mn appears to be present_2.5Co_0.5Ge (x ═ 0.5) compound and Mn₂Further computational studies of the "energy-unfavourable" atomic configuration of the CoGe (x ═ 1) compounds (i.e. systems in which the Co atom is located not only at the T site but also at the "energy-unfavourable" O site) show that the M values of these systems (open symbols without lines in fig. 4) are intermediate between the M values of the corresponding tetragonal and cubic systems in the lowest energy configuration. Note that in Table I, for 0<x is less than or equal to 1, and the energy difference (E) between the cubic structure and the tetragonal structure_cub-E_tet) Relatively small and close to the thermal energy of the film growth temperature (about 0.04 eV). These results show that our Mn_3-xCo_xGe films have a significant degree of atomic disorder. This is a common phenomenon for heusler compound films and can strongly affect many of their properties both in vivo and at the interface.

FIG. 5A shows for different values of x

Mn of thickness_3-xCo_xOut-of-plane XRD theta-2 theta measurements of Ge films. The dashed vertical lines labeled "four squares" and "cubes" are guides for line of sight. FIG. 5B summarizes the c lattice extracted from FIG. 5AThe parameters correspond to the corresponding values from the DFT computation. The experimental c-lattice constants extrapolated from fig. 5A are shown by black open circles, while the theoretical c-lattice constants are represented by filled squares. The cross-hatched area is a guide to the line of sight. Undoped Mn₃Ge film (x ═ 0) exhibits a lattice constant c out of plane_tetIs about

Corresponding 2 theta_tet-(004)Pure tetragonal structure with a (004) oriented bragg peak at about 50.5 °, consistent with previous reports. Mn is also observed as a distinctly characterized spectrum (fingerprint) of alternating layers within the unit cell (consistent with the Mn-Mn and Mn-Ge end layers in fully ordered compounds)₃Ge (002) superlattice peak (theta)_tet-(002)About 24.5 °). When the Co concentration increases to x 0.6, the (004) peak moves linearly to a higher 2 θ angle, indicating a straight drop in the elongated axis of the tetragonal cell (see fig. 5B). Only the film with x ═ 0.6 (dashed line in fig. 5A) shows a mixed phase structure: the (004) bragg reflection corresponding to the tetragonal phase and the cubic phase was found. Further increase in Co content (0.6)<x.ltoreq.2) causes the complete disappearance of the tetragonal peak and the lattice constant c_cubAbout

Cubic phase (2 theta)_cub-(002)31 deg. and 2 theta _cub-(004)65 deg.) with single crystal L2 grown by molecular beam epitaxy on a GaAs (001) substrate (FIG. 5B)₁-cubic MnCo₂Ge heusler films were consistent.

FIGS. 6A-6B show measurements by SQUID-VSM at 300K

Thick Mn_3-xCo_xOut-of-plane (fig. 6A) and in-plane (fig. 6B) M-H hysteresis loops of Ge films. Fig. 7 depicts x 0.25 grown at room temperature (black line), 250 ℃ (dark gray line) and 350 ℃ (light gray line)

Thick Mn_3-xCo_xRoom temperature of Ge filmX-ray diffraction theta-2 theta scan. The structure transition from tetragonal to cubic and the associated loss of symmetry of disruption of the crystal structure upon Co doping contribute to changing the in-plane perpendicular magnetic anisotropy of the film. This is by Mn_3-xCo_xThe Ge films were confirmed with M-H (magnetization versus applied field) hysteresis measurements at 300K for the out-of-plane (fig. 6A) and in-plane (fig. 6B) sample configurations. In FIG. 6A, tetragonal Mn₃Ge (solid black line) film was shown to have a large coercive field (H)_CAbout 3.6T) and low magnetization (M)_SAbout 100emu/cc), and FiM structural features thereof. When the x ≧ 0.16 film (dashed line) shows tetragonal crystal structure, the magnetic moment is too small to show a clear PMA hysteresis loop, but films with higher Co content (x ≧ 0.8) show the expected in-plane magnetic anisotropy and FM configuration.

Alternatively, tetragonal Mn having a c-axis substantially coincident with the film line can be grown on the underlayer_3-xCo_xA Ge film, the bottom layer consisting of a CoAl or CoGa or CoGe or CoSn or Co layer and at least one other element (preferably Al); or Al is alloyed with Ga, Ge, Sn, or any combination thereof, such as AlSn, AlGe, AlGaSn, AlGeSn, and algalgalgesn. These types of underlayers may be referred to as template layers. Fig. 7 shows Mn of 0.25 at x grown at Room Temperature (RT), 250 ℃ and 350 ℃ on a MgO (001) substrate with a CoAl template layer_3-xCo_xOut-of-plane XRD theta-2 theta measurements of Ge films. The materials of these films are stacked as

And (5) Ta. The composition of the CoAl layer is Co_1-yE_yAnd y is in the range of from 0.45 to 0.55. Mn at 2 θ ≈ 51 °_2.75Co_0.25The appearance of the Ge (004) peak indicates the growth of the tetragonal phase of the Hasteller film on the CoAl template layer. The CoAl template layer facilitates the growth of ultra-thin heusler compound films with analogous bulk properties.

The structures described herein make them suitable for a variety of applications including MRAM elements and racetrack memory devices. Examples of such MRAM elements are shown in fig. 8A and 8B. Generally like MRAM elements, the tunnel barrier is located between two magnetic electrodes, one of which has a fixed magnetic moment and the other of which has a switchable magnetic moment, allowing the recording and erasing of data. Unlike prior art MRAM elements, however, the magnetic layers of fig. 8A and 8B, which have either a fixed magnetic moment (the pinned layer) or a switchable magnetic moment (the storage layer), include heusler films such as the heusler described herein. An optional (second) interfacial layer may be advantageously selected for better performance, as described herein.

Note that in fig. 8A, the pinning layer (heusler layer, which may be ferromagnetic or ferrimagnetic) covers the seed layer, which in turn covers the substrate. As described herein, the respective compositions of the seed layer and the substrate may be advantageously selected to promote growth of a heusler layer having magnetic moments oriented perpendicular to the plane of the layer. Alternatively, the second interfacial layer may be used to improve performance and may comprise Fe, CoFe alloy or Co₂MnSi. Note that in fig. 8B, a switchable layer (heusler layer, which may be ferromagnetic or ferrimagnetic) overlies the seed layer, which in turn overlies the substrate. As described herein, the respective compositions of the seed layer and the substrate may be advantageously selected to promote growth of a heusler layer having magnetic moments oriented perpendicular to the plane of the layer. Alternatively, the second interfacial layer may be used to improve performance and may comprise Fe, CoFe alloy or Co₂MnSi。

Although tunnel barriers of other (001) orientations such as CaO and L iF may be used, the tunnel barrier is preferably MgO (001)₂O₄As a tunnel barrier, the lattice spacing of the tunnel barrier can be adjusted by controlling the Mg — Al composition, which can make a better lattice match with heusler compounds. The magnetic electrode overlying the tunnel barrier may comprise, for example, Fe, CoFe alloy, or CoFeB alloy. Fig. 8B also shows that when the magnetic layer on top of the tunnel barrier has a fixed magnetic moment, its magnetic moment can be stabilized by placing a Synthetic Antiferromagnet (SAF) in its vicinity. The capping layer may comprise Mo, W, Ta, Ru, or combinations thereof. Can be used for dredgingA current is induced by applying a voltage between two magnetic electrodes separated by a tunnel barrier.

Some of the structures described herein may also be used in racetrack memory devices. In this case, the racetrack is a nanowire that may include a substrate, an optional seed layer, a template layer, and a first magnetic layer of heusler compound. (for possible compositions of these layers, see the discussion above with respect to FIG. 8A. Note that in racetrack memory devices, the tunnel barrier and switchable magnetic layer as shown in FIG. 8A will generally not be present; however, in this case, the first magnetic layer shown in FIG. 8A will have a switchable rather than fixed magnetic moment.) the domain walls can move along the racetrack as described in U.S. patent 6834005. Data may be read from (and stored in) the racetrack by interrogating (or changing the orientation of the magnetic moments of) the magnetic material between adjacent domain walls within the racetrack.

The various layers described herein may be deposited by any one or more of a variety of methods including magnetron sputtering, electrodeposition, ion beam sputtering, atomic layer deposition, chemical vapor deposition, and thermal evaporation.

Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive.

Claims (20)

1. A device comprising a tunable tetragonal ferrimagnetic heusler compound, the device comprising:

is Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x ≦ 1, wherein Co comprises at least 0.4 atomic percent of the Hasteller compound, and the Hasteller compound has a tetragonal structure; and

substrate, oriented in (001) direction and in YMn_1+dForm wherein Y comprises an element selected from the group consisting of Ir and Pt, and 0. ltoreq. d.ltoreq.4,

wherein the heusler compound and the substrate are in proximity to one another such that a spin-polarized current passes from one of the heusler compound and the substrate through the other of the heusler compound and the substrate.

2. The device of claim 1, wherein Y is Ir.

3. The device of claim 1, further comprising a tunnel barrier in contact with the heusler compound.

4. The device of claim 3, wherein the tunnel barrier comprises Mg and O.

5. The device of claim 1, further comprising a TaN layer between and in contact with the heusler compound and the substrate.

6. The device of claim 1, wherein the heusler compound is Mn_3-xCo_xForm Ge, wherein 0<x is less than or equal to 1, and the substrate is IrMn₃Form (a).

7. The device of claim 1, wherein the heusler compound has a magnetic moment perpendicular to a plane of the substrate.

8. The device of claim 1, wherein the heusler compound has a thickness of at least 10 angstroms and no more than 500 angstroms.

9. The device of claim 1, further comprising a TaN layer underlying and in contact with the substrate.

10. The device of claim 1, wherein the heusler compound has a thickness of less than 5 nm.

11. The device of claim 1, wherein the heusler compound has a thickness of less than 3 nm.

12. A device comprising a tunable tetragonal ferrimagnetic heusler compound, the device comprising:

is Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x ≦ 1, wherein Co comprises at least 0.4 atomic percent of the Hasteller compound, the Hasteller compound having a tetragonal structure;

a multilayer structure that is non-magnetic at room temperature, the multilayer structure comprising alternating layers of Co and E, wherein E comprises at least one other element that comprises Al, wherein the composition of the multilayer structure consists of Co_1-yE_yAnd y is in the range of 0.45 to 0.55; and

a substrate positioned below the multi-layer structure,

wherein the heusler compound and the multilayer structure are in proximity to one another such that a spin-polarized current passes from one of the heusler compound and the multilayer structure through the other of the heusler compound and the multilayer structure.

13. The device of claim 12, wherein the magnetic moment of the heusler compound is perpendicular to an interface between the multilayer structure and the heusler compound.

14. The device of claim 12, wherein the heusler compound has a thickness of one unit cell.

15. The device of claim 12 wherein E is an algaam alloy.

16. The device of claim 12, wherein E comprises an alloy selected from the group consisting of AlSn, AlGe, algalgalgag, AlGaSn, AlGeSn, and algalgesn.

17. The device of claim 12, further comprising a tunnel barrier overlying the heusler compound, such that current passes through both the tunnel barrier and the heusler compound.

18. The device of claim 17, further comprising a magnetic layer in contact with the tunnel barrier.

19. A device comprising a tunable tetragonal ferrimagnetic heusler compound, the device comprising:

a substrate;

a bottom layer oriented in a (001) direction, the bottom layer being non-magnetic at room temperature, the bottom layer covering the substrate;

a first magnetic layer comprising Mn_3-xCo_xHeusler compounds of the Ge form, where 0<x is less than or equal to 1, and the first magnetic layer is in contact with the bottom layer, wherein the magnetic moment of the first magnetic layer is switchable;

a tunnel barrier covering the first magnetic layer; and

a second magnetic layer in contact with the tunnel barrier.

20. The device of claim 19, further comprising a capping layer in contact with the second magnetic layer.

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